1. Field of the Invention
The invention relates to a water-cooled electrode string for electric arc furnaces, and particularly to a composite electrode comprising one or more hollow graphite bodies with a central bore through which a tube carries cooling water exiting the tube into a hollow nipple or electrode and flowing back up through the central bore to an exit at the upper end of the composite electrode. In this invention, a consumable solid graphite electrode is normally at the lower end of the electrode string. If the composite electrode or nipple or the attached counterbored electrode of Ser. No. 573,704 fails in service resulting in a break or split, a large amount of cooling water could quickly flow into the furnace producing a large volume of steam resulting in an explosion. This invention is a safety shut-off acting as a first line of defense during such occurrences.
2. Previous Developments in the Art
The conventional material employed in electrodes for electric arc furnaces is graphite. These electrodes are consumed in use, for example in electric arc steel making furnaces, due to erosion caused by oxidation, sublimation, spalling and other factors. This consumption involves tip losses, column breakage losses and particularly surface oxidation losses. An average electric furnace consumes four to eight kilograms of graphite per metric ton of steel produced.
One method for reducing the consumption of graphite electrodes in arc furnaces has been the application of a protective coating or cladding material to the electrodes with oxidation resistant materials. These coatings generally increase the contact resistance to the electrode power clamp, and some are corrosive, as they are based on phosphoric acid. Consequently, they have not found wide acceptance.
Another means for reducing graphite electrode consumption involves the utilization of fully nonconsumable electrode systems. These systems employ full length liquid-cooled metal electrodes with selected apparatus to protect the electrode from the extreme temperatures of the arc. Although such systems are known, in practice they have not been successful due to high costs and known catastrophic failures in service.
It has been suggested that composite electrodes comprising carbon or graphite portions attached to a water-cooled metallic piece would provide a means for reducing electrode consumption in arc furnaces. A number of patents have issued on specific composite electrode designs. For example, U.S. Pat. Nos. 961,139 to Keller; 896,429 to Becket; 2,471,531 to McIntyre et al.; 3,588,307 to Kegel et al.; 3,392,227 to Ostberg; 4,121,042 and 4,168,392 to Prenn; 4,189,617 and 4,256,918 to Schwabe et al.; 4,287,381 to Montgomery; 4,291,190 to Elsner et al.; and U.S. Pat. No. 4,509,178 by Lades et al. relate to liquid cooled composite electrodes for arc furnaces. Likewise, European Pat. application Nos. 50,682; 50,683; 53,200 by C. Conradty, Nurnberg; 115,812; and 77,513 and by Von Roll AG are directed to composite electrode configurations. These design versions are complex, expensive, and present numerous unsolved operational and manufacturing problems.